It is known that some fluorophthalic anhydrides or acids may be prepared by halogen exchange, that is, fluorination of the corresponding chlorinated phthalic anhydrides. However, success in this approach has varied depending on the degree of halogen substitution on the chlorophthalic anhydride reactant. Thus, 3-chlorophthalic anhydride, 4,5-dichlorophthalic anhydride, and 3,6-dichlorophthalic anhydride have all been reported in the literature to yield the corresponding fluorinated anhydrides upon reaction with potassium fluoride. (Fifolt, M. J. and Foster, A. M., U.S. Pat. Nos. 4,374,266 and 4,374,267; and Bergman, E. D., Bentov, M., and Levy, A. J. Chem. Soc. (1964) 1194). However, under similar conditions, attempts at direct fluorination of higher chlorinated anhydrides have been reported as unsuccessful. Odinokov, V. N., Yakobson, G. G., and Vorozhtsov, Jr. N. N. Zh. Org. Khim. (1967), 37, 176, disclose that the reaction of tetrachlorophthalic anhydride with potassium fluoride in dimethylformamide produced no tetrafluorophthalic anhydride with octafluoroanthraquinone as the only identified product. A similar reaction in the absence of a solvent was reported by the same investigators to yield 40-45% octafluoroanthraquinone. Furthermore, the present inventors have attempted direct fluorination of trichlorophthalic anhydride by reaction with potassium fluoride under varying conditions with extremely low yields (less than 10%).
Ito, H., Matsuschita, U., Shimizu, T., and Ishikowa, N. (U.S. Pat. No. 4,769,493) disclose the preparation of N-substituted tetrafluorophthalimide by reaction of the corresponding N-substituted tetrachlorophthalimide.
Islam, A. M., El-Sharief, A. M. S. and Bedair, A. H. (Indian J. Chem., 16B, July 1978, p. 593-596), disclose the preparation of N-phenyl-triiodophthalimide by reaction of triiodobenzalphthalide with hydrazine hydrate.
It is an object of the present invention to provide a novel and efficient process for the preparation of 3,4,6-trifluorophthalic acid.